High-strength plain carbon steels having improved formability

ABSTRACT

Fully killed high-strength plain carbon steels consisting essentially of 0.06% to .20% carbon, .4% to 1.2% manganese, .005% to .3% silicon, .04% maximum sulfur, .04% maximum phosphorus, an inclusion shape control agent comprising .05% to .20% zirconium, or .01% to .10% of rare earths or .01% to .10% mischmetal, balance iron are characterized in a hot-rolled finished condition by a yield strength in excess of 35,000 p.s.i., an ultimate tensile strength in excess of 55,000 p.s.i., ductility as measured by percent elongation (2 inches) in excess of 30%, good toughness and superior formability. The steels are hot-rolled finished in the temperature range 1550° F. to 1650° F., cooled at a rate within the temperature range 20° F. to 135° F. per second and collected by coiling or piling within a temperature range of 900° F. to 1200° F., preferably between 1025° F. to 1175° F.

This invention relates generally to high-strength steels andparticularly to high-strength plain carbon steels of superiorformability.

We have developed a class of high-strength plain carbon steels which ina hot-rolled finished condition exhibit good ductility and a toughnessand strength previously available only in steels containing an alloystrengthening agent or steels heat treated after hot rolling. Theimproved properties are brought about by hot rolling the steels undercritical temperature conditions. The steels of the invention alsoexhibit superior formability resulting from the use of an inclusionshape-control agent comprising either zirconium, or rare earths, ormischmetal which, of course, is a mixture of rare earths. The use of aninclusion shape-control agent results in the formation of substantiallyspherically shaped inclusion which retain their spherical shape in thehot-rolled finished material.

Accordingly, an object of the present invention is to provide plaincarbon steels having high strength in combination with good toughnessand ductility and superior formability. Another object of the inventionis to provide such steels characterized in a hot-rolled finishedcondition by yield strengths in excess of 35,000 p.s.i., ultimatetensile strengths in excess of 55,000 p.s.i., ductilities as measured bypercent elongation (2 inches) in excess of 30% and good toughness. Stillanother object of the invention is to provide such steels which can bebent without cracking about an inside radius which is equal to or lessthan about one-half the thickness of the steels.

These and other objects and advantages of the present invention willbecome apparent from the following detailed description thereof.

The steels of the present invention are fully killed and have thefollowing general chemistry: carbon, .06% to .20%; manganese, .4% to1.2%; slicon, .005% to .3%; sulfur, .04% maximum; phosphorus, .04%maximum; an inclusion shape-control agent comprising either .05% to .20%zirconium, or .01% to .10% of rare earths or .01% to .10% mischmetal;and balance iron.

The preferred steels of the invention consist essentially of .12% to.16% carbon, .5% to .7% manganese, .1% maximum silicon, .02% maximumsulfur, .03% maximum phosphorus, .08% to .12% zirconium or .01% to .10%of rare earths or mischmetal, balance iron. Rare earths which areemployed in the steels of the invention are, for example, cerium,lanthanum, praseodymium, neodymium, yttrium and scandium.

Steels to possess the desired characteristics and properties of a yieldstrength in excess of 35,000 p.s.i., an ultimate tensile strength inexcess of 55,000 p.s.i., ductility as measured by percent elongation (2inches) in excess of 30% and good toughness are hot-rolled finished inthe temperature range of 1550° F. to 1650° F., cooled at a rate withinthe range of 20° F. to 135° F. per second and collected by coiling orpiling within a preferred temperature range of 1025° F. to 1175° F.Steels finished and/or collected at temperatures in excess of thetemperatures set out above or cooled at a rate less than 20° F. persecond generally exhibit strengths below a yield strength of 35,000p.s.i. and an ultimate tensile strength of 55,000 p.s.i. In addition,the steels do not have as good impact properties as steels hot rolledwithin the temperature ranges set out above. Steels finished or coiledbelow the preferred temperature ranges exhibit ductilities as measuredby percent elongation inferior to the ductilities of steels of theinvention. In addition, low finishing temperatures result in productionliabilities in that rolling speeds must be slower to achieve lowerfinishing temperatures.

As noted above, the inclusion shape-control agents cause the sulfideinclusions in the steels to retain a spherical form, resulting in asignificant improvement in the formability of the material. In theabsence of an inclusion shape-control agent, certain inclusions becomeelongated during hot rolling and aligned parallel to the rollingdirection and adversely affect the formability of the steels.

Sufficient zirconium is added to the steels of the invention so thatthere is a minimum of .02% zirconium in the steel in excess of thezirconium which combines with the nitrogen in the steel to formnitrides. For a typical high-strength low alloy steel containing .006%nitrogen, therefore, approximately a minimum of .06% zirconium is addedto the steel. The minimum amount of zirconium required is given by thefollowing formula: Percent zirconium=0.02% zirconium+6.5 (wt. percentN). The zirconium is preferably added to the steel in the ingot moldduring teeming. Zirconium additions are made when the mold is aboutone-third full and the additions completed by the time the mold is abouttwo-thirds full. Typical zirconium recoveries achieved employing thismethod of addition are about 60%. The zirconium additions can also bemade to the ladle after the heat is tapped. However, the steel in theladle must be fully killed to assure good zirconium recovery. In thistechnique, it is important to employ good teeming practice to minimizeoxygen or nitrogen entrainment during teeming which adversely affectszirconium recovery.

The significance of processing the steels within a finishing temperaturerange of 1550° F. to 1650° F., cooling the steel at a rate within thetemperature range 20° F. to 135° F. per second and collecting the steelwithin a preferred temperature range of 1025° F. to 1175° F. isdemonstrated in Table I.

                                      TABLE I                                     __________________________________________________________________________                                                    Longi-                                   Rolling                              tudinal                                  temperatures                                                                            Cooling    Ultimate                                                                            Percent                                                                            Grain                                                                              impact                                   (° F.)                                                                           rate,                                                                              Yield tensile                                                                             elon-                                                                              size trans.                              Gage Fin- Col- ° F. per                                                                    strength,                                                                           strength,                                                                           gation                                                                             (ASTM                                                                              temp. (°F.)            Product                                                                             inch ish  lected                                                                             second.sup.1                                                                       p.s.i.                                                                              p.s.i.                                                                              (8") No./)                                                                              (50% FATT)                    __________________________________________________________________________    Coil  0.250                                                                              1,630                                                                              1,050                                                                              40   40,900                                                                              64,700                                                                              28.0 9.6  -50                                 0.312                                                                              1,610                                                                              1,070                                                                              35   38,900                                                                              66,200                                                                              29.5 10.4 -55                           Plate 0.250                                                                              1,790                                                                              1,300                                                                              2    35,600                                                                              62,400                                                                              26.5 8.9  -25                                 0.312                                                                              1,760                                                                              1,350                                                                              2    32,700                                                                              60,000                                                                              31.0 7.6  -15                           __________________________________________________________________________     .sup.1 Cooling rate between the finishing and collecting temperature.    

The heat from which the products of Table I were processed had ananalysis of .14% C, .16% Mn and .06% Si. The lower finishing and coilingtemperatures of the coiled products together with their more rapidcooling rates resulted in their smaller grain size and higher strengthand improved notch toughness, as shown by their lower fractureappearance transition temperatures (FATT), as compared with the plateproducts.

The improved properties brought about by the increased cooling rates ofthe invention are shown in Table II.

                                      TABLE II                                    __________________________________________________________________________                                         Longi-                                                                        tudinal                                                                       impact                                                        Ultimate                                                                            Percent   trans.                                                  Yield tensile                                                                             elon-                                                                              Grain                                                                              temp.                                    Heat  Gage,                                                                             (°F./                                                                       strength,                                                                           strength,                                                                           gation                                                                             size (°F.) (50%                        No.   inch                                                                              seconds)                                                                           p.s.i.                                                                              p.s.i.                                                                              (1") (ASTM)                                                                             FATT)                                    __________________________________________________________________________    427829                                                                              0.50                                                                              2    30,300                                                                              51,600                                                                              36.5 6.6  >+70                                           0.50                                                                              66   47,700                                                                              63,700                                                                              36.5 8.8  -20                                      COM-2 0.50                                                                              2    33,300                                                                              55,000                                                                              37.5 7.0  +70                                            0.50                                                                              73   47,600                                                                              65,900                                                                              37.5 9.4  -10                                      __________________________________________________________________________

Heat No. 427829 was a plain carbon steel containing .14% carbon, .28%manganese and .06% silicon. Heat No. COM-2 was a plain carbon steelcontaining .12% carbon, .44% manganese and .05% silicon. The fastercooling rates resulted in reduced ferrite grain sizes which bring aboutthe higher yield strengths and improved toughness.

The improved formability of the steels of the invention is shown inTable III.

                                      TABLE III                                   __________________________________________________________________________                                                             Minimum                                                                       bend                                                                          radius                                                 Shelf energy           without                       Chemistry (weight percent)                                                                       Yield ft.-lbs. in                                                                              50% ductile-brittle                                                                       cracking             Gage,                       strength,                                                                           1/2 size charpy                                                                          transfer temp.,                                                                           transverse           Heat No.                                                                           inch                                                                              C Mn Si S  P  Zr                                                                              Al p.s.i.                                                                              V-notch specimens                                                                        (°F.)                                                                              sample               __________________________________________________________________________    957220                                                                             0.280                                                                             .19                                                                             .39                                                                              .042                                                                             .020                                                                             .007                                                                             --                                                                              .030                                                                             42,600                                                                              Longitudinal, 54                                                                         Longitudinal,                                                                              1T                                                    Transverse, 17                                                                           Transverse, -10                  957007                                                                             0.250                                                                             .14                                                                             .50                                                                              .010                                                                             .019                                                                             .010                                                                             .08                                                                             .035                                                                             40,500                                                                              Longitudinal, 61                                                                         Longitudinal,                                                                             .2T                                                    Transverse, 38                                                                           Transverse,                      __________________________________________________________________________                                                 -60                          

The improved bending properties of material from Heat No. 957007,containing zirconium as a shape-control agent, is demonstrated by thefact that steels from that heat could be bent about an inside radius ofa minimum of .2 inch of their thickness without cracking, whereas steelsfrom Heat No. 957220, which did not contain an inclusion shape-controlagent, could only be bent about a minimum inside radius equal to itsthickness before cracking. Crack lengths less than 0.10 inch werediscounted. The table further shows that the zirconium contributed toimproved toughness, particularly in the transverse direction. Equivalentimproved formability and toughness is obtained using rare earths ormischmetal rather than zirconium as the inclusion shape-control agent.

We claim:
 1. A killed high-strength plain carbon steel which has beenhot-rolled finished in the temperature range 1550° F. to 1650° F.,cooled at a rate within the range of 20° F. to 135° F. per second, andcollected within a preferred temperature range of 1025° F. to 1175° F.,the steel being characterized in a hot rolled condition by a yieldstrength in excess of 35,000 p.s.i., an ultimate tensile strength inexcess of 55,000 p.s.i., ductility as measured by percent elongation (2inches) in excess of 30%, good toughness and formability, said steelconsisting essentially of .06% to .20% carbon, .4% to 1.2% manganese,.005% to .3% silicon, sulfur in an amount up to .04%, .04% maximumphosphorus, a sulfide inclusion shape-control agent selected from thegroup consisting of .05% to .20% zirconium, .01% to .10% of a rare earthand .01% to .10% mischmetal, balance iron, the sulfide inclusions in thesteel having a substantially spherical shape.
 2. The steel of claim 1wherein the sulfide inclusion shape-control agent comprises .05% to .20%zirconium.
 3. The steel of claim 1 wherein the sulfide inclusionshape-control agent comprises .01% to .10% of rare earths.
 4. A killedhigh-strength plain carbon steel which has been hot-rolled finished inthe temperature range 1550° F. to 1650° F., cooled at a rate within therange of 20° F. to 135° F. per second, and collected within a preferredtemperature range of 1025° F. to 1175° F., the steel being characterizedin a hot rolled condition by a yield strength in excess of 35,000p.s.i., an ultimate tensile strength in excess of 55,000 p.s.i.,ductility as measured by percent elongation (2 inches) in excess of 30%,good toughness and formability, said steel consisting essentially of.12% to .[..15%.]. .Iadd..16% .Iaddend.carbon, .5% to .7% manganese,.[..3%.]. .Iadd..1% .Iaddend.maximum silicon, sulfur in an amount up to.02%, .03% maximum phosphorus, a sulfide inclusion shape-control agentselected from the group consisting of .08% to .12% zirconium, .01% to.10% of a rare earth and .01% to .10% mischmetal, balance iron, thesulfide inclusions in the steel having a substantially spherical shape.5. The steel of claim 4 wherein the sulfide inclusion shape-controlagent comprises .[..05% to .20%.]. .Iadd..08% to .12%.Iaddend.zirconium.
 6. The steel of claim 4 wherein the sulfideinclusion shape-control agent comprises .01% to .10% of rare earths.